There are numerous problems in the field of plant for blowing packaging, for example bottles, made of synthetic materials. First of all, it is necessary to have a gas, such as air, at a high pressure of the order of 40 bar in order to blow the packaging in a mold. To compress the gas use is conventionally made of compressors. However, in certain blowing methods, the preform is first of all pre-blown to convert it into a packaging and then undergoes actual blow-molding proper. The pre-blowing is performed at a pressure of around 10 bar, then the blowing reaches a pressure of 40 bar. One first way of obtaining these two working pressures is to use two compressors, one to generate the 10-bar pressure and the other to generate the 40-bar pressure. However, the use of two compressors is expensive both in terms of initial outlay to purchase them and in terms of the maintenance they require.
Another option is to use just one compressor compressing the gas to 40 bar, then to expand some of the compressed gas to 10 bar for the pre-blowing operation. This method is not very advantageous either because energy is wasted compressing a gas whose pressure is reduced without having used it in the manufacturing process.
In addition, once the packaging has been formed by blowing, the compressed gas used (which is at a pressure of about 40 bar) is released to the open air and this constitutes a waste of energy.
Hence, it has proved necessary to improve the known methods and machines in order to enhance their efficiency and reduce wasted energy.
Machines and methods for the manufacture of packaging, particularly bottles made of PET, by blowing are known from the prior art. French Patent Application 2 662 631 for example describes such a machine and such a method. According to the method described, a preform fixed to an air distributor is brought into a mold, the distributor is connected with a source of pressurized air, said air being introduced into the preform in such a way as to press the walls of the preform against the edges of the mold, thus forming the desired packaging. In the method described in that application, use is also made of a stretching ram, whose action, combined with that of the pressurized air, gives self-regulation over the balance between axial and transverse deformation of the preform resulting from a combined stretching and blowing method. In the machine described in that patent application, the pressurized air blown into the preform is also used to operate the stretching ram. Once the packaging is formed, the blown air is discharged to the open air.
The anterior publication WO 96/25285 describes another method for blowing packaging, such as bottles for example, from a preform. According to the known method, a pre-blowing of the preform at a pressure of between 8 and 12 bar is performed, then the actual blowing is performed by injecting air at 40 bar. In that document also, the blowing gas is discharged to the open air once the packaging has been formed.
Patent Application EP 0 039 929 describes a method for blowing molded items in which the blowing gas is recovered and recycled for reuse.
More specifically, in that application, the blowing gas also has a function of treating the interior surface of the blown packaging, particularly to improve the impermeability of said interior surface, and for that purpose it contains an appropriate reagent. Once the blowing and treatment have been performed, the blowing gas is recovered for subsequent reuse, by means of a purging gas, said purging gas itself having to be purified of the reagent used.
Patent Application EP 0 655 313 describes a method and a device for recovering blowing air. In the method described, air at high pressure used for blowing a container is recovered directly in the low-pressure source. This has the disadvantage of creating pressure variations in the low-pressure source and these detract from the correct formation of the container. In addition, the blown container is held for a short length of time at the high blowing pressure, and this too detracts from the good quality of the blown container.
U.S. Pat. No. 5,173,241, finally, describes a method and an apparatus for recovering the blowing air in a plastics injection molding machine. In this prior art, the blown air is recovered in the recovery container by suction and is reintroduced into the system's compressor.